As semiconductor devices scale down to nano-scale dimensions (i.e., ≦100 nm features), the performance improvements predicted by Moore's Law typically diminish. Some fundamental physical properties, such as direct quantum tunneling through gate dielectrics, poly depletion in gate electrodes and source-drain leakage due to short channel effects, may become limiting factors in performance and may inhibit further scaling of conventional devices.
One method to suppress short channel effects in field effect transistors is to make the semiconductor channel of the transistor sufficiently thin that it becomes fully depleted during operation. One such device is a planar thin-body semiconductor-on-insulator (SOI) transistor, where the source-drain leakage current can be controlled by a high quality back oxide. However, the thin-body SOI transistor may suffer from poor drain-induced barrier lowering and significant short channel threshold voltage reduction due to reach-through of the drain field through a bulk oxide region. The dual-gate MOSFET with both a front gate and a back gate can provide an effective solution to the problems encountered by thin-body SOI transistors. An extension of the dual-gate transistor is the surround gate transistor, which includes a gate that wraps around (i.e., surrounds) the channel.
Several double/surround gate devices that have been proposed can be classified as either horizontal devices or vertical devices. In horizontal devices, the gate length can be defined using photolithography techniques. However, conventional photolithography techniques may not efficiently scale to nano-scale dimensions. In vertical devices, conventional photolithography techniques may not be required to achieve nano-scale dimensions. For example, in vertical field effect transistors, the gate length and other features may be defined by film thickness instead of a photolithographically defined line width. Conventional vertical devices, such as surround gate transistors, are disclosed in an article by E. Leobandung et al. entitled “Wire-Channel and Wrap-Around-Gate Metal-Oxide-Semiconductor Field-Effect Transistors with a Significant Reduction of Short Channel Effects,” J. Vac. Sci. Technol., B 15(6), pp. 2791-2794, November/December (1997). Vertical devices are also disclosed in an article by C. Auth et al., entitled “Scaling Theory for Cylindrical, Fully-Depleted, Surrounding-Gate MOSFET's,” IEEE Elec. Dev. Lett., Vol. 18, No. 2, pp. 74-76, February (1997).
Additional devices that utilize nano-scale metal or semiconductor materials may be formed using nanowires. When wires fabricated from metal or semiconductor materials are provided in the nanometer size range, some of the electronic and optical properties of the metal or semiconductor materials at nano-scale dimensions may be different from the same properties of the same materials at a larger scale. Semiconductor structures in the nanometer size range that exhibit the characteristics of quantum confinement are typically referred to as zero-dimension (OD) quantum dots or more simply as quantum dots when the confinement is in three dimensions. Quantum dots may be provided by semiconductor materials having one or more dimensions on the scale of about ten nanometers or less. When quantum confinement is in two dimensions, the structures are typically referred to as one-dimensional quantum wires or more simply as quantum wires. A quantum wire is a wire having a diameter sufficiently small to cause confinement of an electron gas in directions that extend normal to the wire.
A prior art technique for fabricating quantum wires may utilize a micro-photolithographic process followed by a metalorganic chemical vapor deposition (MOCVD) process. This technique may be used to generate a single quantum wire or a row of gallium arsenide (GaAs) quantum wires embedded within a bulk aluminum arsenide (AlAs) substrate. However, such techniques may not be compatible with processes to form two or three dimensional arrays of nanowires in which the spacing between nanowires is relatively small and uniform.
Additional techniques for forming two-dimensional arrays of nano-channels include filling naturally occurring arrays of nano-channels or nanopores in a substrate with a material of interest. In this manner, the substrate is used as a template. Exemplary substrates include anodic aluminum oxide and mesoporous materials, which may be provided with arrays of pores therein. In particular, U.S. Pat. No. 6,359,288 to Ying et al. discloses techniques for forming arrays of nanowires in anodic aluminum oxide substrates. One of these techniques includes systematically changing the channel diameter and channel packing density of an anodic aluminum oxide layer by anodizing an aluminum layer with an electrolyte to provide an anodic aluminum oxide layer having nanopores therein. The mean pore diameter is disclosed as varying by no more than 100% along the length of the pore. The '288 patent also discloses filling the pores with single crystal material so that the resulting nanowires constitute single crystal quantum wires. These quantum wires may have an average wire diameter in a range of about 1 nm to about 20 nm. U.S. Pat. No. 6,231,744 to Ying et al. also discloses a method of forming a nanowire array by anodizing an aluminum substrate using an acidic electrolyte solution to provide a porous aluminum oxide film (i.e., anodic aluminum oxide (AAO) film) on a surface of an aluminum substrate. The porous AAO film is then exposed to an acid etchant solution for a period of time sufficient to enlarge the cell size of the pores.
Techniques for forming porous films and nano-scale electronic devices are disclosed in European Patent Specification No. EP 0 178 831 B1 and in U.S. Pat. No. 6,034,468 to Wilshaw. In particular, the '468 patent to Wilshaw discloses a field emitter device having a dielectric AAO layer therein with nanopores. The front ends of the wires constitute individual field emitting cathodes. A gate electrode is also provided on a front surface of the AAO layer. U.S. Pat. No. 5,581,091 to Moskovits et al. also discloses single-electron devices that are useful as diodes and transistors. These devices are prepared by anodizing a metal substrate in an acid bath to convert the metal substrate into an oxide film.